Catalytic reforming is one of the most widely used refining processes, upgrading a low-octane naphtha boiling range fraction to a high octane gasoline blending stock. The reforming reactions are carried out in the presence of hydrogen and include isomerization, dehydrocyclization, dehydrogenation and hydrocracking. In particular, alkanes are both isomerized to iso-alkanes and dehydrocyclized to naphthenes. These newly formed naphthenes together with those existing in the feedstream are then dehydrogenated to aromatics. To summarize, reforming reshapes a low-octane naphtha feed into a product stream which more closely resembles a high-octane mixture of iso-octane and toluene.
Gasoline yield, octane increase and reaction severity are interrelated in the reforming process. Higher reaction temperatures and lower space velocities provide a greater octane upgrade but carry the penalty of increased light gas and coke production. In addition to decreasing the relative gasoline yield, additional coke make accelerates catalyst deactivation. Further, the incremental product shift from liquid to gas loads the reformer gas plant and decreases the economic value of the overall product slate.
For example, U.S. Pat. No. 3,890,218 to Morrison teaches a reforming process using a crystalline zeolite catalyst having the structure of ZSM-5. The Morrison patent shows a plot of C.sub.5 + volume percent recovery as a function of research clear octane number for a given feed and process conditions. This limitation inherent to catalytic reforming is clearly incompatible with the present and future product demands.
Recent changes in internal combustion engine design to enhance fuel efficiency and performance while decreasing pollutant emissions have increased demand for high octane gasoline. Demand is growing not only for a greater volume of premium gasoline but also for premium gasolines having higher octane numbers. Further, stricter environmental regulations preclude the use of many highly efficacious octane enhancing additives such as tetraethyl lead. Refiners must now meet the market demand for higher octane gasoline in relatively greater volumes without the use of prohibited octane enhancing additives. At the same time, refiners must meet the need for additional hydrogen to supply new hydroprocessing units, examples of which include catalytic dewaxing and lube hydrotreating units.
By way of background, catalytic reforming process units are typically associated with a produce recovery section, or gas plant. The gas plant includes a plurality of fractionation towers and may also include one or more gas/liquid absorption towers. An example of a typical gas plant scheme includes a debutanizer, depropanizer and deethanizer/absorber.
Reformer reactor effluent typically flows first to feed/effluent preheat exchangers and then to air or water cooled reactor effluent coolers. The cooled reactor effluent is flashed in a high pressure separator vessel to split off hydrogen-rich recycle gas from the reactor effluent product which is, at this point, called unstabilized reformate.
Unstabilized reformate is then charged to the debutanizer fractionator which separates the C.sub.4 - fraction as overhead product from C.sub.5 + normally liquid gasoline product. The C.sub.4 - debutanizer fractionator overhead product is then charged to a depropanizer which fractionates the debutanizer overhead into a butane-rich bottom stream and a C.sub.3 - overhead stream. Finally, the C.sub.3 - stream is cut into a propane-rich bottom stream and an overhead light gas stream which is usually burned as fuel gas.
Catalytic processes have also been developed to convert light C.sub.2 -C.sub.4 hydrocarbons to more valuable C.sub.5 + compounds, particularly C.sub.6 + aromatics. These processes are commonly referred to as M-2 Forming and are described in the article, "M2 Forming-A Process for Aromatization of Light Hydrocarbons" by N.Y. Chen and T.Y. Yan, 25 IND. ENG. CHEM. PROCESS DES. DEV., 151 (1986), the teachings of which are incorporated herein by reference for a general overview of such catalytic upgrading processes.
Commercial reforming units with their widely recognized record of reliable operation have become cornerstones of modern gasoline refining facilities. These reformer units would benefit from an improvement which combines their relatively high throughput with enhanced liquid product yield and octane number.
U.S. Pat. No. 3,928,174 to Bonacci et al. teaches a process for improving the overall product slate from a catalytic reforming process by catalytically upgrading C.sub.2 - light gas to a mixture of C.sub.3 -C.sub.4 aliphatics commonly referred to as LPG. Thus, the reforming process may be operated to maximize production of C.sub.5 + aromatic gasoline and the resulting C.sub.2 - light gas, rather than being burned as fuel gas, is converted into saleable LPG product.
Further developments in the art of light hydrocarbon upgrading, specifically aromatization of light hydrocarbons, are exemplified by U.S. Pat. No. 4,347,395 to Chu et al. which discloses a process for the conversion of C.sub.2 -C.sub.16 paraffinic hydrocarbons to aromatics in the presence of oxygen and a zeolite catalyst. Still further advances in M-2 Forming are taught in U.S. Pat. Nos. 4,579,987 to Chang et al; 4,590,321 to Chu; 4,607,130 to Chang et al; and 4,629,818 to Burress.